1. Field of the Invention
The present invention relates generally to recording apparatuses and, more particularly, to a recording apparatus including a laser beam scan optical device and having a recording density alterable by changing the intensity of a laser beam.
2. Description of the Related Art
Recently, as recording apparatuses such as of laser beam printers and facsimile devices, various apparatuses incorporating laser beam scan optical devices for use in image writing have been proposed. In such a laser beam scan optical device, a laser beam emitted from a light source (mainly a semiconductor laser) is deflected within one plane by a deflector (a polygon mirror, hologram scanner, galvano mirror or the like), then guided onto a photoreceptor being a recording medium via an f-.theta. lens, a reflecting mirror or the like and is imaged. During this process, a part of the laser beam irradiates a light receiving element (hereinafter referred to as an SOS sensor), whereby a formed signal showing the position to start image printing on each scanning line (hereinafter referred to as a horizontal synchronizing signal) is obtained. After a certain period of time has passed since this horizontal synchronizing signal was output, a writing of image information onto the photoreceptor is started, thereby synchronizing respective lines. Also, as the above-described recording apparatus, such a recording apparatus has been proposed that an image having a different recording density can be reproduced with a change in the density of dots imaged on a photoreceptor by changing the intensity of a laser beam.
FIG. 11 is a diagram showing the intensities of beam incident on the SOS sensor and a horizontal synchronizing signal generated by each beam. As shown in FIG. 11, differences in the intensities of the laser beams directed to the SOS sensor cause differences in output waveforms of the SOS sensor and a deviation in the horizontal synchronizing signal.
More specifically, an output of the SOS sensor has waveforms which are different in rising and falling due to the difference in beam intensities, as shown in FIG. 11 (a). Even if this signal is shaped at a predetermined threshold value in order to obtain a horizontal synchronizing signal, deviations .delta.1 and .delta.2 occur in the horizontal synchronizing signal due to the difference in beam intensities, as shown in FIG. 11 (b). Thus, when image writing is started after a certain period of time, with the rising or falling of the horizontal synchronizing signal being set as a reference, for example, a so-called printing deviation corresponding to the deviation .delta.1 or .delta.2 is produced. Alternatively, ever if a middle point between the rising and the falling of the horizontal signal is set as a reference, a printing deviation is produced corresponding to (.delta.1+.delta.2)/2 due to the difference between .delta.1 and .delta.2. Accordingly, in a recording apparatus having a recording density variable by a variation in beam intensity, the positions for image writing are not uniformed, thereby causing.